The present invention relates generally to fuel supply and control apparatus for gas burners. More specifically, it relates to a gas valve system of the type having solenoid operated pilot and main valves and an electronic control circuit for achieving fail-safe gas flow control.
Fuel gas valves of the type having a pilot valve and a main valve are used in many applications. In the residential market, for example, gas fueled space heaters, water heaters, stoves, and ovens commonly use a pilot valve/main valve arrangement as these devices are typically operated only intermittently. For environmental and economic reasons, recent designs of these systems include electronic ignition of a pilot burner when use is called for, rather than continuous burning of the pilot flame.
It is obviously desired to use gas system components which reduce as much as possible the risk of explosions and/or other hazards resulting from unintended gas release. One common level of protection against such occurrences is to employ a valve design in which the main valve is arranged in series with the pilot valve. This assures that there can be no gas flow through the main burner if gas is not available to the pilot burner. A second level of protection can be provided by testing for the actual existence of or proving the pilot flame with a thermocouple or other device. In such an approach, a signal from the thermocouple causes the actuation of the main valve only when the pilot flame has been proved.
Fail-safe operation of the above arrangements, however, depends on proper operation of the electronic and/or electrical controls. A worn or broken relay may, for example, open a valve or hold open a valve when such a result is unwanted. In response to these concerns, redundant parts or relay logic may be used to prevent unintended gas release caused by failure of components in the gas valve system.
Fail-safe systems which utilize relays and/or redundant mechanical parts, however, suffer from larger size and increased cost. Relay logic also consumes an undesirably large amount of power for operation. A further problem with such systems relates to long term reliability and operability. Mechanical components are more prone to deterioration and failure, and hence likely to have a shorter useful life than functionally equivalent solid state implementations.
Solid-state control systems address many of the disadvantages of mechanical control systems. Specifically, solid-state systems require less operating power and generally have longer functional lifetimes. Furthermore, solid-state devices tend to be smaller and less costly to build.
Use of solid-state gas valve control systems with fail-safe mechanisms have been addressed in a number of patents. For example, U.S. Pat. No. 5,085,574(Wilson), describes a solid-state, fail-safe gas valve system in which failure of any single part in the safety circuit prevents actuation of a gas valve. U.S. Pat. No. 4,865,538(Scheele etal.) describes another solid-state, fail-safe gas valve which discriminates against false enabling signals such as a direct current signal, line voltage signal, or non-repetitive alternating current signal.
Both patents prevent actuation of the controlled valve due to system failure by using an intermediate charge-storing device and a switching means. The switching means is situated to prevent current flow from the charge storage device to the valve without proper operation of each part in the control system. Neither patent, however, suggests how to apply the method described to a pilot valve/main valve system.
The present system overcomes the disadvantages discussed above in known fail-safe main valve/pilot valve systems. It provides a cost-effective, solid-state, fail-safe gas valve drive circuit which eliminates problems with mechanical relay failure and the higher initial and maintenance costs of electromechanical relay based approaches. Further, it requires less operating power, in part because there is no requirement for power to maintain relay actuation.